Download Naming the Carboxylic Acids

19-1
Naming the Carboxylic Acids
Chemical Abstracts retains the common names for the two simplest carboxylic
acids, formic acid and acetic acid.
The IUPAC system derives the name of carboxylic acids by replacing the ending –e
in the parent alkane by the ending –oic acid.
The alkanoic acid stem is numbered by assigning 1 to the carbonyl carbon and
labeling any substituents along the longest chain incorporating the CO2H group
accordingly.
The carbonyl group and the functional groups of its derivatives take precedence in
naming over any other groups discussed so far:
When other functional groups are present, the main chain is chosen to include
other functional groups as much as possible.
Saturated cyclic acids are named as cycloalkanecarboxylic acids.
Aromatic acids are named benzoic acids.
Dicarboxylic acids are referred to as dioic acids.
19-2
Structural and Physical Properties of Carboxylic Acids
Formic acid is planar.
The molecular structure of formic acid is roughly planar, which is characteristic of
carboxylic acids in general.
The carboxy group is polar and forms hydrogen-bonded dimers.
The carboxy function is strongly polar and forms hydrogen bonds to other
polarized molecules such as water, alcohols and other carboxylic acids.
Carboxylic acids up to butanoic acid are completely soluble in water.
As neat liquids, and even in fairly dilute solutions, carboxylic acids form hydrogenbonded dimers (6–8 kcal mol-1).
Carboxylic acids have relatively high melting and boiling points due to hydrogen
bonding in both the solid and liquid states.
19-3
NMR and IR Spectroscopy of Carboxylic Acids
The carboxy hydrogen and carbon are deshielded.
Hydrogens on a carbon next to a carbonyl group are slightly deshielded. The effect
diminishes rapidly with increasing distance from the carbonyl.
The hydroxyl proton resonates at very low field ( = 10-13 ppm). Its chemical shift varies
strongly with concentration, solvent and temperature because of its involvement in
hydrogen bonding.
The 13C NMR chemical shifts of carboxylic acids are similar to those of aldehydes
and ketones. The amount of deshielding is smaller because of the presence of the
extra OH group.
The smaller deshielding can be attributed to the extra resonance form present in
carboxylic acids:
The carboxy group shows two important IR bands.
Stretching frequencies for both the carbonyl group and the hydroxy substituent
are seen in the IR spectra of carboxy groups.
The O-H bond exhibits a very broad band at 2500–3300 cm-1, lower than for
alcohols because of strong hydrogen bonding.
Mass spectra of carboxylic acids show three modes of fragmentation.
Fragmentation of carboxylic acids occurs in several ways, which results in a fairly
weak molecular ion peak.